(1) Field of the Invention
This invention relates to antenna systems which electronically control antenna beams; and, in particular, to such antenna systems capable of producing nulls in their radiation patterns.
(2) Prior Art
A multiple beam antenna can be formed using an array of antenna elements, a lens, or a reflector, and can have an adjustable amplitude and/or phase control of each constituent beam. These adjustable phase and amplitude controls are set to provide an excitation in accordance with a desired radiation pattern. That is, the multiple beam antenna can have a composite far field radiation pattern which is adjustable, and thus provide variable coverage.
A multiple beam antenna pattern may be formed with one or more narrow, deep depressions, often termed nulls, within a wide, flat-top shaped beam antenna pattern. Generating nulls is particularly advantageous to minimize the deleterious effects of discrete sources of interfering radiation which impinge on the antenna aperture while the antenna provides radio communication in other directions. In satellite communications it may be desirable to aim a null at a jamming source while maintaining coverage of the antenna beam pattern in other angular regions.
For example, in "Optimization of a Communication Satellite Multiple Beam Antenna" written by A. R. Dion, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Mass., Technical Note 1975-39, May 27, 1975, pages 20 through 41, it is taught how to form a beam pattern by adjusting the phase and amplitude excitation of all ports equally except for one, two or three contiguous ports, whose composite beam points in the direction desired for a null. In each desired null direction the appropriate composite beam port excitation is adjusted in amplitude and/or phase so that each such composite beam cancels the residual field of the other remaining, uniformly excited ports in the desired null direction. The null is obtained by a subtraction of equal amplitude fields.
A generalized prior art Multi-Beam Antenna is shown in FIG. 1. It consists of an aperture (which could be an offset parabolic reflector, a microwave lens, or a phased array), a beam connection region (e.g. feed horns at the focal region of a reflector, a lens or a hardwired multibeam array), and multiple beam ports, i.e., one port for each secondary pencil beam. A conical cluster of N overlapping pencil beams is associated with a set of N beam ports. To form a single controllable shaped beam, an N-port-to-one-port Beam Forming Network (BFN) provides control of relative amplitude and phase of the energy being summed from the N beam ports to the one final shaped beam port. Multiple shaped beams may be produced by establishing the proper interconnections and excitations within the BFN.
FIG. 2 shows typical radiation patterns of a set of multiple beams, together with a typical composite pattern formed by simple summation of individual beams in the same phase. If all beams, for example, of a triangular grid of 61 beams are driven simultaneously, beam excitation may be specified by means of a beam excitation diagram as shown in FIG. 3, wherein an arrow in each circular beam contour indicates excitation phase by twist angle and excitation magnitude by arrow length. The magnitude of the twist angle is measured in a clockwise direction from a vertical reference line. FIG. 3 shows the case of equal phase and amplitude of all 61 beams. The excitation of FIG. 3 results in a nearly flat-topped low-ripple, wide shaped beam as in FIG. 4. To communicate with stations in the flat top of the beam, it may be desirable to minimize discrete reception, or radiation, of signals in other discrete directions, such as point A of FIG. 5, by modifying beam excitations so as to create a deep narrow pattern depression (termed a "null") centered on point A. If the null excitations are optimized at a frequency f.sub.o then at other frequencies f.sub.1, f.sub.2, etc., progressively displaced from f.sub.o, changes in sidelobes of constituent pencil beams (singlets) generally spoil the null at point A, causing increasing null fill-in as in FIG. 5.
Referring to FIG. 6, there is shown a simplified drawing of a field subtraction excitation technique for nulling consisting of reversing the phase of a narrow beam, or a contiguous group of beams, such as a doublet (two beams) or a triplet (three beams). The subtraction technique next involves reducing the reversed beam's amplitude so that the peak of the narrow subtracting beam just cancels the residual sidelobe fields from the other co-phased beams. The angular point of cancellation forms the bottom of the desired pattern null. When the null is positioned within a single beam (e.g., A or B in FIG. 6) the field subtraction null process can be done with that single beam. For a null centered between two contiguous singlets (e.g., C and D), the CD doublet is driven equally but reduced in amplitude, with phase adjusted to cancel residual sidelobe fields of the other beams. Similarly, three beams (e.g., E, F and G) can be driven together and phase reversed to create a subtraction null at the center of the triplet beam. Two noncontiguous nulls within the flat topped beam can be achieved by the use of beams at both nulls (e.g., A and B) which are each out of phase with the local residuals at the null. Further, if needed, nulls can be steered to any point in a local unit cell about the singlet, doublet or triplet null point by unequal excitation of the composite subtraction beam amplitudes.
Other prior art inclues a publication by Kiyo Tomiysu entitled "Sequential Phasing in Multiple Beam Antenna for Interference Reduction", 1977 IEEE AP-S Symposium Digest, pp 428-431 which discloses steering of a null by subtraction with uneven excitation of 2, 3 or 4 contiguous beams. Beams surrounding the null are given a sequential increasing phase excitation. The purpose of this sequential phase excitation of adjacent beams is to be able to use a simple control algorithm to steer the null covered by the three beams while reducing the residual sidelobes from surrounding coverage beams. That is, the particular phase and amplitude of the surrounding beam excitations is used to pull down and hold down the level of their sidelobes so as to simplify phase control of the nulling composite beam. The article does not disclose any particular arrangement of excitation phases so as to reduce the effect on the nulls of changing the frequency of operation.
If there is a requirement such as maintaining a minimum desired gain over the anenna beam coverage over a 4% bandwidth, and requiring that the coverage include a null of specified depth, it has been determined that it is more difficult to maintain the null using prior art techniques than to maintain the gain over the required bandwidth. That is, it is relatively easy to maintain the gain of the antenna beam pattern over the 4% bandwidth and relatively difficult to maintain a sufficiently deep null over the 4% bandwidth using the previously described subtraction technique.
Although the prior art teaches a way of providing a null in a specified composite pattern, such as the subtraction technique discussed above, there are often undesirable side effects such as a change in the composite pattern in other directions, and a change in the composite pattern as a function of frequency, wherein a null is frequency dependent. It has been a problem in communication systems to provide a desired null depth over relatively wide frequency bandwidths for a fixed set of beam port excitations. These are some of the problems this invention overcomes.